1. FIELD OF THE INVENTION
The present invention relates to a method for preparing a 3-methyl-2(4'-halogenophenyl)-butyronitrile.
2. DESCRIPTION OF THE PRIOR ART
In the conventional alkylation of phenylacetonitrile, phenylacetonitrile is reacted with an alkyl halide, in the presence of an alkali metal, alkali metal halide, alkali metal amide or the like as a base, in an anhydrous aprotic solvent or liquid ammonia. Details of the reaction conditions and yields are summarized in Organic Reactions Vol. 9, 297 (1957). However, many difficulties are encountered in handling large amounts of the above-mentioned bases so that this conventional method is not always advantageous for large scale industrial production. Furthermore, the yield and purity of the .alpha.-mono-alkylated phenylacetonitriles obtained are unsatisfactory.
Alkylation of the .alpha.-position, which has been recently developed, proceeds via the sodium salt of .alpha.-phenylacetoacetonitrile with an appropriate alcohol [Journal of Organic Chemistry Vol. 37, 526 (1972)]. This method gives relatively high yields when the alkyl groups introduced into the .alpha.-position are a straight chain alkyl group having more than 6 carbon atoms, but the yield is extremely low with lower or branched alkyl groups. Further, this method is not advantageous for industrial scale production since pressurized reaction vessels, such as an autoclave, are required in a case of lower alcohol. In recent years, a new alkylation process in which an organic quaternary ammonium salt is used as a phase transfer catalyst has been found, but for the reasons mentioned hereinafter this process is not completely acceptable.
A. Brandstrom et al. have reported [Tetrahedron Letters 473, (1972)] that when phenylacetonitrile is methylated with methyl iodide in methylene dichloride in the presence of a 7-10% aqueous sodium hydroxide solution using tetra-n-butyl-ammonium hydrogen sulfate (n-Bu.sub.4 N.sup.+HSO.sub.4 .sup.-) in an amount equimolar to the acetonitrile, the .alpha.-methylated product (72%), .alpha., .alpha.-dimethylated product (14%), and the starting material (14%) are obtained as a mixture at a yield of 84%. When the alkylating agent is isopropyl iodide or ethyl iodide, the monoalkylated product alone is obtained at conversions of 75% and 90%, respectively, but 10 to 25% of the phenylacetonitrile used as the starting material remains unreacted. This method has two serious disadvantages: first, the alkyl iodide is expensive; second, on an industrial scale, such conversion levels make it necessary to separate the objective product from the resulting mixture, i.e., to use a separation procedure such as rectification. Furthermore, when the substituent at the .alpha.-position is a lower alkyl group, as desired by the inventors, even separation by a low cost technique such as rectification is not accessible.
M. Makosza et al. also studied the alkylation of phenylacetonitrile using quaternary ammonium salts as a catalyst, but satisfactory yields and purity could not be obtained. For example, they reported in Roczniki Chem. Vol. 39, 1223 (1965) and Chemical Abstracts Vol. 64, 12595 (1965) that when phenylacetonitrile is alkylated at room temperature with ethyl chloride in the presence of a 50% aqueous sodium hydroxide solution using benzyltriethyl ammonium chloride as a catalyst, 2-phenylbutyronitrile is obtained at a yield of 90%. The purity of the product was not disclosed, however.
For the alkylation of phenylacetonitrile where the benzene ring is substituted, it has been reported [Roczniki Chem. Vol. 42, 1619 (1968) and Chemical Abstracts Vol. 70, 37413 (1969)] that 4-bromophenylacetonitrile can be alkylated with .alpha.-chloroacetonitrile under the conditions used to obtain the .alpha., .alpha.-di-cyanomethylated product at yields of 80%.
The inventors formerly investigated alkylation processes using organic quaternary ammonium salts as a catalyst, particularly for the isopropylation of a 4-halogenophenylacetonitrile, a important intermediate for an .alpha.-substituted phenylacetate (as disclosed in U.S. patent application Ser. No. 378,301, filed July 11, 1973), a novel new insecticide discovered by the inventors, which application is hereby incorporated by reference.